Quick answer: Sowthistle is a tall annual weed with milky sap, clasping leaves and yellow dandelion like flowers that turn into fluffy, wind blown seed. UC IPM says it reproduces only by seed. Pull or hoe young plants, never let one finish flowering, and keep bare soil covered with mulch so drifting seed finds less open ground.

What Is Sowthistle?

A fast growing annual in the sunflower family. The University of California Statewide IPM Program (UC IPM) calls annual sowthistle a widespread winter or summer annual broadleaf of agricultural land and disturbed sites. Its habitat list includes crop fields, orchards, yards, gardens and roadsides. In mild areas it grows year round.

NC State Extension describes the close relative, spiny sowthistle, as a spring or summer annual that is often considered a weed because it spreads aggressively. It grows in full sun on many soil types and turns up in lawns, waste places and roadsides. Our overview of weed control in outdoor cannabis names sowthistle among the wind blown edge weeds that land on bare soil.

How Do You Identify Sowthistle?

Snap a stem. UC IPM says annual sowthistle has a smooth, thick, hollow stem that releases a milky sap when cut. Other features from UC IPM:

• Height. Coarse, upright plants that can reach about 4 3/5 feet.
• Leaves. Hairless and somewhat bluish green. Lower leaves are about 4 to 8 inches long and deeply lobed, with a large end lobe.
• Leaf base. Upper leaves are smaller, stalkless and clasp the stem with clawlike lobes.
• Flowers. Daisylike heads at the stem tips that look like urns when closed.
• Seed. Tiny flattened fruits, about 1/10 to 1/6 inch long, each with a tuft of soft white hairs.

The seed heads are white and fluffy, but UC IPM notes they are not ball shaped like a dandelion’s.

How Is Spiny Sowthistle Different?

Touch the leaf edge. NC State says spiny sowthistle leaves have soft but very spiny teeth, wavy margins, a white midvein and large round lobes that clasp the stem and curl downward. The leaves are often dark green and glossy, and can reach 10 inches long, though they are usually about half that size.

Annual sowthistle leaves are softer and more bluish. Both plants have yellow flowers, milky sap and wind carried seed, so you manage them the same way. If you already know the dandelions in cannabis beds, note that sowthistle grows a tall leafy stem with several flower heads, while a dandelion holds one head on a bare stalk.

Why Does Sowthistle Matter Near Cannabis?

It moves fast, and it travels far. NC State says spiny sowthistle can bolt and form flower heads very quickly in summer, and warns gardeners not to let it go to seed because the seed spreads by wind and water and the plant can take over a site. It flowers from June to August, and its seeds ripen from July to September.

UC IPM adds that annual sowthistle can harbor pests of vegetable and fruit crops, including several aphids and nematodes, some of which can carry plant viruses. Keeping it out of beds and edges removes one more host plant from the space around your cannabis.

When Does Sowthistle Grow?

Almost any time it gets the chance. UC IPM says annual sowthistle can be a winter or summer annual, with flowering mostly in spring and summer and year round under good conditions. That means seedlings can appear in fall cleanups, in early spring beds and in summer gaps between plants.

UC IPM’s landscape guide says the best time to look for winter annual weeds is mid to late winter, and summer annuals are easiest to spot in mid to late summer. For sowthistle, check both times, plus any bare patch after rain. Our guide to fall garden cleanup after cannabis harvest is a good moment to pull fall seedlings.

Can Sowthistle Seed Blow In From Elsewhere?

Yes. NC State says the seed spreads by wind and water, and each seed of spiny sowthistle carries a tuft of silky white hairs that the wind picks up. Plants on a nearby fence line or vacant lot can add seed to your beds.

You cannot stop every seed, but you can stop it from finding open ground. UC IPM’s landscape guide says mulch prevents weed seed germination by blocking sunlight, and that new plantings should cover bare areas quickly. Soil that stays covered gives drifting seed fewer places to start.

How to Control Sowthistle in a Cannabis Garden

1. Learn the stem test. Milky sap and a hollow stem mark annual sowthistle, as UC IPM describes.
2. Pull or hoe young plants. UC IPM’s landscape guide says to remove young weeds by hand before they flower and set seed.
3. Wear gloves for spiny sowthistle. NC State says its leaves carry soft but very spiny teeth.
4. Never let a plant finish blooming. NC State warns gardeners not to let spiny sowthistle go to seed.
5. Bag plants that already flowered. Remove them from the garden rather than leaving them on the bed, so their seed cannot spread.
6. Check edges and fence lines. NC State says the seed spreads by wind and water, so clear nearby sources too.
7. Mulch bare soil. UC IPM recommends a medium mulch kept 3 to 4 inches deep. Our guide to mulch for cannabis compares materials.

Sowthistle at a Glance

Subject	Extension finding	Attribution
Life cycle	Winter or summer annual	UC IPM, NC State
Mature height	Up to about 4 3/5 feet	UC IPM
Key trait	Hollow stem with milky sap	UC IPM
Leaf base	Upper leaves clasp the stem	UC IPM, NC State
Seed spread	Wind and water	NC State
Reproduction	By seed only	UC IPM

Should You Use Herbicides Near Cannabis?

No. Pull, hoe shallowly and mulch. If herbicides ever come up for other parts of your property, follow the label exactly. Remember that EPA has not registered any herbicide for use on marijuana, and state rules vary, so check with your state agriculture department. UC IPM says home gardeners who hand weed and mulch should not need herbicides. Before you plant cannabis outdoors, check your state’s home grow law.

Troubleshooting Sowthistle

New plants keep appearing in clean beds. NC State says the seed travels on wind and water. Mulch bare soil and clear nearby edges.

Flowering plants were left on the bed. Bag them and remove them from the garden so their seed cannot spread.

Plants bolted while you were away. NC State says spiny sowthistle can bolt and flower quickly in summer. Check beds often in warm weather.

You cannot tell it from prickly lettuce. Both have milky sap. UC IPM lists prickly lettuce as a related plant. Manage both the same way.

Hands get scratched. NC State notes the spiny leaf teeth. Wear gloves and grip near the base.

At Kind Seed Co, we see a clean bed edge as the first line of defense against wind blown weeds.

Frequently Asked Questions

Is sowthistle an annual or a perennial?

Annual sowthistle and spiny sowthistle are both annuals. UC IPM calls annual sowthistle a widespread winter or summer annual, and NC State calls spiny sowthistle a spring or summer annual. Both reproduce by seed. A related plant, perennial sowthistle, also exists, but the two common garden types finish their life in one season.

How can I tell sowthistle from a dandelion?

Sowthistle grows a tall, leafy, hollow stem with several yellow flower heads, and UC IPM says it releases milky sap when cut. A dandelion holds a single flower on a bare stalk above a low rosette. UC IPM also notes that sowthistle seed heads are fluffy but not ball shaped like a dandelion’s.

Is spiny sowthistle harmful to touch?

Wear gloves. NC State says spiny sowthistle leaves have soft but very spiny teeth along wavy margins, so bare hands can get scratched. Grip the stem near the soil when pulling it. The plant is managed the same way as annual sowthistle, by pulling it before seed forms.

When should I pull sowthistle?

Pull it as soon as you see it, and always before it flowers. NC State says spiny sowthistle flowers from June to August and its seeds ripen from July to September, and UC IPM says annual sowthistle can grow year round in mild areas. Check regularly through spring and summer and remove plants before seed forms.

Where does sowthistle grow?

UC IPM lists crop fields, orchards, vineyards, pastures, yards, gardens, roadsides and disturbed sites as annual sowthistle habitat. In a home garden, that means bare bed edges, paths and fence lines. Its fluffy seed drifts in from those spots, so mulch nearby bare soil and pull plants before they flower, as UC IPM’s landscape guide advises.

Sources

• UC Statewide IPM Program, Annual Sowthistle, Weed Gallery: https://ipm.ucanr.edu/weeds-identification-gallery/annual-sowthistle/
• NC State Extension Gardener Plant Toolbox, Sonchus asper: https://plants.ces.ncsu.edu/plants/sonchus-asper/
• UC Statewide IPM Program, Weed Management in Landscapes: https://ipm.ucanr.edu/home-and-landscape/weed-management-in-landscapes/